Epoxidation of Isobutylene

Isobutylene oxide is produced in a way similar to propylene oxide and butylene oxide by a chlorohydrination route followed by reaction with Ca(OH)2. Direct catalytic liquid-phase oxidation using stoichiometric amounts of thallium acetate catalyst in aqueous acetic acid solution has been reported. An isobutylene oxide yield of 82% could be obtained. 

Direct non-catalytic liquid-phase oxidation of isobutylene to isobutylene oxide gave low yield (28.7%) plus a variety of oxidation products such as acetone, ter-butyl alcohol, and isobutylene glycol  

Hydrolysis of isobutylene oxide in the presence of an acid produces isobutylene glycol  

Isobutylene glycol may also be produced by a direct catalyzed liquid phase oxidation of isobutylene with oxygen in presence of water. The catalyst is similar to the Wacker-catalyst system used for the oxidation 

Liquid-phase oxidation of isobutylene glycol produces othydroxyisohu-tyric acid. The reaction conditions are 70-80°C at pH 2-7 in presence of 

---

In marked contrast to the generally accepted mechanism, the involvement of a radical pair produced by an alkene-induced 0—0 bond homolysis was suggested by Minisci and coworkers . In a combined experimental and theoretical study Curci, Houk and coworkers sought to differentiate between a radical pathway and the commonly accepted concerted mechanism. Both product and kinetic smdies tended to exclude a radical pathway. Computational studies at the B3LYP/6-31G level on the epoxidation of isobutylene with DMDO predicted an activation energy = 15.3 kcalmor ) significantly lower... 

These composite data strongly suggest that the presence of adventitious water or other hydrogen donors can markedly affect the observed rate of epoxidation. For example, Murray and Gu have reported AH = 5.0 kcalmol" for the DMDO epoxidation of cyclohexene in CDCI3 and 7.4 kcalmol" in acetone as solvent . Curci and coworkers also reported an a value of 9.3 kcalmol" for the DMDO epoxidation of isobutylene in acetone . These barriers are significantly lower than the 13-18 kcalmoD gas-phase barriers reported " at the B3LYP level of theory (Tables 3 and 4). Activation barriers of 12.6,... 

FIGURE 22. Selected geometrical parameters of the transition stmcture for the epoxidation of isobutylene with peroxyformic acid calculated at the QCISD/6-31G, CISD/6-31G (in parentheses), B3LYP/6-31G (in square brackets), B3LYP/6-311+G(3df,2p) (in italic in square brackets) and MP2/6-31G (in curly brackets) levels... 

Puskas, J.E., Brister, L.B., Michef A J., I-anzenddrfer, M.G., Jamieson, D., and Pattern, W.G. Novel substituted epoxide initiators for the carbocationic pol3mierization of isobutylene, 7. Polym. Set, 38,444-451, 2000. Puskas, J.E. and Michel, A.J. New epoxy initiators for the controlled synthesis of functionalized polyisobutylenes, Makromol. Chem., Macromol. Symp., 161, 141-148, 2000. 

The epoxidation of propylene is discussed in Chapter 10, Section 2. Some isobutane can be made by isomerizing -butane. The isomerization of -butenes to isobutylene is also being commercialized. 

The numbers in brackets for propylene, isobutylene, iJ-2-butene and 1,3-butadiene entries are at the QCISD(T)//QCISD/6-31G(d) level of theory QCISD(T)/6-31G(d)//B3LYP/6-311- -G(3df,2p) gas-phase intrinsic barriers (AE ) for the epoxidation of -2-butene with dimethyldioxirane (DMDO) and peroxyformic acid are 14.3 and 13.2 kcalmol respectively. 

The epoxidations of propylene and isobutylene with peroxyformic acid proceed in a concerted way via slightly unsymmetrical Markovnikov-type transition stmctnres where the differences in the bond distances between the donble-bond carbons and the spiro oxygen are only 0.021 and 0.044 A at the QCISD/6-31G level. In contrast, the more polarizable natnre of the carbon-carbon double bond of o ,/ -unsaturated systems results in a highly nnsymmetrical transition structure for the epoxidation of 1,3-butadiene with an order-of-magnitnde difference in the carbon-oxygen bond distances of 0.305 A at the QCISD/6-31G level. A highly unsymmetrical transition structure has been also found for the epoxidation of acrylonitrile. 

The sequence of reactivity for the butenes is ds-2-butene (16) > 1-butene (6) > isobutylene (4.7) > trans-2-butene (1.0). A higher reaction rate for the cis isomer was also observed in the epoxidation of 2-hexenes, as was retention of stereochemical configuration (Tatsumi et al., 1990a). 

There are two possible types of mechanism for the uncatalyzed hydrolysis of epoxides, a simple SN2 reaction of the substrate with water and a reaction of the protonated substrate with hydroxide ion. Another question to be answered concerns the position of attack of the nucleophile in substituted ethylene oxides. Experiments by Long and Pritchard [150] with H2180 indicate that in the uncatalyzed hydrolysis of propylene oxide two-thirds of the overall reaction occur via attack at the primary carbon. The corresponding percentage for the reaction of isobutylene oxide has not been determined precisely, but it is 20 % at least, probably much more. Attack at the primary carbon predominates also in the uncatalyzed reaction of propylene oxide with chloride ion [152]. 

Several conclusions are drawn from the product studies of the reactions of propylene oxide and isobutylene oxide in H O.23 In the acid-catalyzed hydrolysis of propylene oxide, cleavage of the secondary C-O bond is favored by a factor of 2-3 over cleavage of the primary C-O bond. In the acid-catalyzed hydrolysis of isobutylene oxide, cleavage of the tertiary C-O bond is highly favored (>99%). These results show that in acid-catalyzed epoxide hydrolysis, cleavage of the C-O bond leading to... 

Song, J., Bodis, J., and Fhiskas, J.E. Direct functionalization of poly isobutylene by living initiation with alpha-methylstyrene epoxide, J. Polym. Set, Polym. Chem., 40, 1005, 2002. 

Chen, Y., Puskas, J.E., and Tomkins, M. Investigation of the effect of epoxide structure on the initiation efficiency in isobutylene pol3mierizations initiated by epoxide/TiCLt systems, Eur. Polym. J., 39, 2147-2153, 2003. 

---